Leakproofing article for building use

ABSTRACT

The present invention concerns a manufactured article impervious to water, the water being rain water, water coming from a water-bearing stratum, and/or from concrete. The manufactured article of the invention may be used in building of habitations, factories, tunnels, and/or submarine structures and the like, the article comprising a leakproofing material. The manufactured article further includes a coating constituted by a film made of a material, the water-solubility of which is kept under control, the film delaying the leakproofing material hydration process for a predetermined period of time from between 1 and 30 days.

TECHNICAL FIELD

The present invention relates to the technical field of building ofhabitations and/or factories, as well as of more complex building workssuch as building of tunnels, submarine structures and the like.

More particularly, the present invention aims to provide for aleakproofing article to be used in the building of habitation and orfactory structure foundation, or for building of underground structuresand/or tunnels, as well as in the more difficult technical field ofsubmarine works.

The present invention may be mainly applied in the general technicalfield of building.

BACKGROUND ART

The use of bentonite as leakproofing material in the field of buildingis well known in the art.

For instance, it is used in the building of foundation and/or in anyother type of application in which a suitable degree of imperviousnessto water has to be assured.

It is also known that sodium natural bentonite is a clay which undergoesa substantial hydration when it comes into contact with water, saidhydration increasing the bentonite volume up to thirty times its dryvolume.

As a consequence of the hydration process, it is known that thebentonite produces both a gel phase of the colloidal type, which causesan increase of the bentonite volume of about 10-15 times with respect toits dry volume, and a solute phase, which causes an even greaterincrease of the bentonite volume.

Bentonite is used in the form of particles, generally placed inside ofone or more layers of corrugated card-board sheets or of needlednon-woven fabrics, obtaining a structure similar to a panel and/or asmall mattress which is set at the extrados of underground structuresfoundation and/or at any other location where a leakproofing barrier isneeded.

A further solution known the art provides for the use of bentonitecoupled with a suitable binder, e.g. butylic rubbers and/orwater-soluble glues.

In this case, the system binder/bentonite is given a rolled shape or apanel type shape to so that it can be sufficiently handy and easy to belaid.

For instance, U.S. Pat. No 4,048,373 discloses a solution of the typecited above according to which a leakproofing panel is produced, saidpanel being formed by two sheets, advantageously card-board sheets,suitably spaced in order to contain bentonite together with a suitableadditive.

In general, said additive is constituted by a water-soluble dispersant,e.g. a water-soluble salt of acetic or phosphoric or boric acid,combined with a water-soluble polymer, for instance polyacrylic acid,water-soluble salts of polyacrylic acid, polyacrylnitril,polyvinylacetate, polyvinylalcohol or copolymer thereof, according todefined percentages of the components of the bentonitic composition.

According to a preferred form of embodiment disclosed in U.S. Pat. No.4,048,373, the leakproofing panel is constituted by two corrugatedcard-board sheets and by a corrugated paper band which is placed betweenthem.

Said paper band comes into contact with the two sheets forming aplurality of voids which are filled with the bentonitic compositionmentioned above.

U.S. pat. No. 4,048,373 discloses a water impervious panel comprisingone upper and one lower sheets, said sheets being made of one of one ofa range of polymers. A shortcoming of the above document is that thedissolution rate is not predetermined and controlled, theref ore theremay be found to occur the expansion of the bentonite pack at undesiredtimes, and the only thing that can be relied upon in that respect iscarrying out the operations in an extremely expedite way, which is notalways possible whereas it is always to the detriment of accuracy andsafety, and does not rule out t he risk that the expansion takes placewhen the panels are still being worked upon.

Moreover, the panel disclosed in the above document is not structurallyfeasible, as the water swellable material is simply laid upon its solidsupport, therefore it is prone to structural failure.

In the present application the bulk impermeable material is found to bespread over an inner layer made of biodegradable kraft paper that hascharacteristically such a shape that it confers the device an overallinherent resilience and resistance, therefore a particular suitabilityfor use in the building industry as there are no risks for it to sufferfrom structural failures even when the water swellable material has notset.

Furthermore, the dissolution rate of the coating can be controlled withextreme precision.

EP-A-567,692 discloses a clay-based impermeable material to be used inthe building industry, made of sodium or calcium bentonite, and appliedto a support, said material fulfilling its function on account of thefact that the sodium or calcium bentonite is coated with a degradableand temporarily water resistant coating, made of cellulose. Thedurability of the coating disclosed in the above document depends on itsthickness, as after contact with water, it breaks down at a constantrate. Therefore if a material with a relatively long temporary surfaceimpermeability is desired, then encumbering volumes of the surfacecoating must be used, thing which in certain cases may turn out to bedetrimental for the stability of the whole building, as, afterdissolution, free voids are left to bear considerable weights. Anothershortcoming of the disclosure of the above document is that because thebentonite is simply laid upon its support, it is inherently of limitedresilience and resistance.

In other words, this document does not disclose any means for keepingunder control the water-solubility of the material constituting thecoating layer of the leakproofing article.

U.S. Pat. No. 4,565,468 discloses a device for providing a moistureimpervient barrier comprising a base sheet member with a bentonite layeron its upper surface, and a top sheet member positioned over thebentonite.

The top sheet member is constituted of a material pervious to water soas to permit hydration of the bentonite.

The present invention aims to solve the problem constituted by a quick,premature and uncontrollable hydration undergone by granular sodiumbentonite when it contacts water, which can be rain water and/or watercoming from a water-bearing stratum or from concrete, during particularstructure building steps.

Said casual and premature bentonite hydration is often extremelynegative as regards to the final result of the structure which has to bebuilt and to the realization of an effective leakproofing barrier.

In fact, if the bentonite hydration process occurs before the concretestructure is laid, and the obtained structure can result as beingdefective; the consequent effects deriving therefrom can be deleteriousfor the building.

On the other hand, if the bentonite hydration occurs when the concretehas just been cast, the increase in the bentonite volume can exert aharmful pressure on the concrete which does not have a sufficiently highmechanical resistence yet, thereby causing permanent damages to thebuilding structure.

Furthermore, it may happen that the hydrate bentonite, i.e. the expandedbentonite, soils and muddies the concrete reinforcements causingadhesion problems which determine the iron being uncoupled from theconcrete, which phenomenon is known under the name of reduction or zerosetting of the iron/concrete matrix.

Thus, said drawbacks produce remarkable and often irreversible damagesboth to the leakproofing system and to the structure which has to bebuilt.

This means that works must be stopped and a new leakproofing barriermade of bentonite is needed to be laid, thereby causing a loss of timeand an increase of the costs.

The realization of water-soluble plastic and completely biodegradablefilms, e.g. films based on polyvinylic alcohol (PVOH), to be used invery different applications, is also known in the art.

For instance, said articles can be advantageously used as protectivecolloids of numerous chemical components, particularly in paper andtextile fields; as light polarizing filters; as temporary sizes intextile yarns production; as main components in the production of theenvelopes of medicinal capsules; as supports for photosensitive elementsof television screens.

DESCRIPTION OF THE INVENTION

The present invention aims to set aside the disadvantages and drawbackswhich are typical of the background art and to provide for a suitableleakproofing article to be used both in the building of the foundationof habitation and/or factory structures, and in the realization ofcomplex works, such as tunnels, submarine structures and the like.

This is obtained by means of a leakproofing article having the featuresdisclosed in claim 1.

The dependent claims describe particularly advantageous forms ofembodiment of the leakproofing article according to the presentinvention.

The present invention allows to produce a leakproofing article of anysuitable shape and dimensions according to any specific need, theleakproofing function being carried out, for instance, by means ofgranular sodium bentonite acting as active leakproofing element.

According to the present invention, the granular sodium bentonite isintroduced into a specific envelope constituted by a film made ofwater-soluble material, said film being able to delay the start of thebentonite hydration process for a suitably long time which is previouslyfixed on the base of particular building requirements.

The leakproofing effect described above is advantageously achieved byusing bentonite and/or bentonitic compositions, but any otherleakproofing material can be used to carry out the present invention.

According to a further form of embodiment, ion exchange resins and/orcolloid modifiers, e.g. carboxymethylcellulose, can advantageously bemixed to said leakproofing article.

According to the present invention, the bentonite or the bentoniticcomposition, used to obtain a leakproofing barrier, becomes insensitiveto rain water and/or water-bearing stratum permeations and/or to thewater coming from the concrete for the period of time which is neededfor laying the leakproofing barrier and for carrying out the subsequentconcrete casting operation, thereby avoiding the drawbacks cited above.

Said temporary imperviousness to water is obtained by coating thebentonite with a film made of water-soluble material, said film beingable to shield the bentonite from water for a predetermined period oftime (for instance from 1 up to 30 days after the occured contact withwater).

In such a way the bentonite hydration process, and the consequentactivation of the leakproofing effect, can occur in the most suitableinstant of time, thereby avoiding damages to the building structure sothat the effectiveness of the leakproofing barrier is not compromised.

According to the present invention a water-soluble film advantageouslyusable to achieve the above-cited object of the invention is constitutedby an article known under the trademark HYDROLENEO LTF, produced byIDROPLAST S.p.A., Italy.

HYDROLENE® LTF is a water-soluble and biodegradable film with athermoplastic composition consisting of about 75% of polyvinylalcohol athigh molecular weight and average hydrolysis degree (86-88%), while theremaining 25% consists of polyoilic plasticizers and process additives,notably of organic nature.

Generally, said plasticizers are aliphatic high-boiling polyoilsselected from the group comprising natural or synthetic alcohols and/oralcohol-ethers containing at least three carbon atoms per molecule andat least two primary or secondary alcoholic functional groups.

HYDROLENE® LTF water-solubility is controlled by acting on a pluralityof parameters.

First of all, it is necessary to remark that the HYDROLENE® LTFwater-solubility rate strongly depends on the temperature of the watercontacting it.

For instance, FIG. 4 shows the flow of the water-solubility rate withrespect to the temperature in the case of a type of HYDROLENE® LTF.

This curve points out a particular temperature value, called WSTtemperature (Water Solubility Temperature), which divides the graph intotwo distinct zones (zone A and zone B), said value representing thetemperature at which HYDROLENE® LTF completely solubilizes in water.

As it is shown in FIG. 4, the HYDROLENE® LTF water-solubility rateincreases very slowly with the temperature in zone A; on the contrary,when the WST value is reached, the curve passes from zone A to zone Bwhere the HYDROLENE® LTF water-solubility rate quickly increases withtemperature.

A curve similar to that of FIG. 4 can be obtained taking intoconsideration the HYDROLENE® LTF water-solubility rate compared to time.

Thus, when both the water temperature and the period of time after whichthe hydration have to occur are fixed, to keep working inside of zone Ait is necessary to modify the chemical formulation of HYDROLENE® LTF byusing plasticizers and/or shield agents, even combined together.

In fact, said components retain the oxydrilic groups of thepolyvinylalcohol allowing the starting time of the polymeric chainsbreaking reaction to be controlled, while the temperature controls thereaction kinetics.

In order to keep working in zone A, a suitable plasticizer in a givenquantity (e.g., on the base of its viscosity and its bindingproperties), as well as a suitable shielding agent have to be selected.

Furthermore, having defined with (1)

—(CH2-CHOH)n—(CH2-CHOCOCH3)m—  (1)

a generic formulation of a polyvinylalcohol obtained by a controlledhydrolysis of the polyvinylacetate, in which formulation n indicates thealcoholic groups and m indicates the acethylic groups, it is possible tocontrol the water-solubility of said polyvinylalcohol, remarkablylowering it, by using alcohols having n/m rate values higher than 95/5.

Other water-soluble articles based on polyethylene, polycaprolactone,maize amid and other articles similar to HYDROLENE® LTF, i.e. having apolyvinylalcoholic base, can be used as envelopes for the leakproofingmaterial.

As said before, the leakproofing article according to the presentinvention can be obtained in different dimensions, thicknesses andshapes in order to be suitable for any building type and requirement.

According to a first form of embodiment, said leakproofing article has apanel-like shape having standard dimensions in accordance with thespecific use it is addressed to.

Said panel it is constituted by an inner layer, advantageously made ofbiodegradable kraft paper having a honey-comb shape in order to containthe bentonite and/or the bentonitic composition and to maintain ituniformly distributed on the whole panel surface.

This uniformity is very important since the granular sodium bentonite ischaracterized by a very variable granulometry which requires a givenweight for square meter dosage of the bentonite (e.g. Kg/m²) in order toobtain a gel omogeneously distributed on the whole surface to be madeimpervious to water when the hydration process has occured.

Said gel should advantageously be distributed on the surface to be madeimpervious as uniformly as possible; actually, thanks to a complete andimmediate adhesion to the concrete, it determines the formation of agel/concrete matrix which prevents a migration of water to theconcrete/leakproofing interface.

Moreover, if a mistake occurs during the laying operation of thebarrier, or in the case where fissures are formed in the concrete, thegel adhesion to the concrete allows the point where the loss has occuredto be exactly singled out, and in this way it is possible to quickly andefficiently operate in a simple and safe way on the damaged area.

The weight dosage of the bentonite allows granular sodium bentonite tobe used, having any granulometric distribution.

Thus, in a simple and precise way it is possible to obtain leakproofingarticles having a different total permeability rate (K) since, using abentonite of the same quality, said rate is proportional to thebentonite weight for each square meter of surface to be made imperviousto water.

Besides, it is also possible to obtain articles having a differentthickness on the base of the bentonite quantity which is used, therebyrealizing articles having a suitable permeability relative to thespecific hydraulic gradient.

Furthermore, the inner layer honey-comb structure of this panel-shapedform of embodiment allows the bentonite to be kept uniformly distributedwhen the panel is vertically placed or during its transport and/or whenassemblying the leakproofing barrier.

Moreover, the honey-comb structure gives the panel the necessarymechanical strength for being used.

Furthermore, said inner layer is wrapped on both sides by means of awater-soluble plastic film which is suitably formulated and treated soas, once it contacts water at ambient temperature, said film dissolvesafter a predetermined period of time on the base of the characteristicaverage technical times of any building structure.

Said water-soluble plastic films are finally coupled, for protectionpurposes, to a mechanical reinforcement element which is respectivelymade of non-woven fabric on the dry side directed to the surface to bemade impervious, and in biodegradable kraft paper or in thermoformednon-woven fabric on the side which directly contacts the externalenvironment.

The layer which is placed on the dry side, and thus directed towards thesurface to be made impervious, is advantageously made of thermoformednon-woven fabric based on low molecular weight polyester, and has asuitable open structure in order to allow the passage of the hydratedbentonite, i.e. the bentonite in its gel phase.

According to a further form of embodiment, said layer made of non-wovenfabric can be replaced by a fabric layer having suitable weft, warpingand weight.

Said gel, passing through said non-woven fabric and permanently buryingthe latter, realizes in this way the structure/leakproofing matrix citedabove.

Moreover, said layer gives the underlying water-soluble film amechanical protection, notably during the panel transport and/orassemblying operations.

According to the present invention the layers constituting the panel areadvantageously joined together by using a water-soluble glue havingphysical-chemical characteristics which are equivalent to those of thewater-soluble plastic film wrapping the honey-comb shaped inner layer.

Furthermore, it is possible to realize the coupling of the layersconstituting the panel by using non water-soluble glues provided thatthey are not continuously and uniformly spread.

This means that the non water-soluble glue layer must be distributed insuch a way as to leave some free areas on the surface interested to theglueing operation, said free areas allowing the passage of the water andof the bentonite as disclosed above.

When said layers are superimposed to each other according to the citedsequence, the panel is then completed by sealing the longitudinal andcross edges of the water-soluble plastic films by thermal welding and/orglueing by means of water-soluble and/or non water-soluble glues.

The panel-shaped form of embodiment according to the present inventionis particularly used as leakproofing barrier in building of undergroundstructures, tunnels and any other complex structures, such as submarineworks.

Said panels can also have different shapes, they do not compulsory showa square cross-section, and they can also be very long (up to a hundredmeters or more).

A further form of embodiment of the present invention provides for aarticle having a kerb or bead shape, which is particularly suitable forthe impermeabilization of casting shut joints.

According to said form of embodiment, pure bentonite and/or bentoniticcomposition and/or any other leakproofing material in powder form and/oras a mixture, is filled inside of an envelope which advantageously has acylindrical shape.

Said envelope is made of water-soluble plastic material which isleakproofing for a predetermined period of time as previously describedwith reference to the panel-shaped form of embodiment.

These envelopes can be made of different and specific dimensionsaccording to the different building and/or application needs, and theyare then produced in small dimensions (generally not longer than onemeter), or wrapped in rolls up to some meters long.

Said kerbs are cheap and easy to be placed, e.g. by using fasteningelements made of thermoformable plastic material or of electric weldedmetallic nets.

ILLUSTRATION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent by reading the following description, given as a non-limitingexample, with the help of the figures illustrated in the attacheddrawings, in which:

FIG. 1 shows a schematic section view of a first form of embodiment ofthe article according to the present invention;

FIG. 2 shows a second form of embodiment of the article according to thepresent invention, and

FIG. 3 shows a method for fastening an article according to the form ofembodiment of FIG. 2.

FIG. 4 shows a graphical representation.

DESCRIPTION OF A FORM OF EMBODIMENT

In FIG. 1, reference sign 10 indicates a leakproofing article accordingto a first form of embodiment of the invention, in which said articlehas the shape of a panel.

Said panel is constituted by an inner layer 11 made of biodegradablekraft paper and having a honey-comb shape in order to contain and keepuniformly distributed a material 40, advantageously bentonite, on thewhole panel surface.

Moreover, said honey-comb shape of the inner layer 11 allows thebentonite to be kept uniformly distributed also when the panel isvertically placed or during its transport and/or assemblying operations.

Said shape also confers to the panel the necessary mechanical strengthin order to assure a sufficient rigidity in use.

Besides, said inner layer 11 is wrapped on both sides by means of a filmthe water-solubility of which is kept under control, said film beingadvantageously constituted by HYDROLENE® LTF 12 and 13, which dissolveswhen contacting water only after a predetermined period of time which ispredetermined on the base of the type of structure to be built.

Said films 12 and 13 are coupled, for protection purposes, to amechanical reinforcement element which is respectively made of eithernon-woven fabric or fabric 14, on the dry side directed to the surfaceto be made impervious to water, and of biodegradable kraft paper 15 onthe side which directly contacts the external environment.

The layer 14 is made of thermoformed non-woven fabric, constituted bylow molecular weight polyester, and it has an open structure in order toallow the passage of the bentonite during its gel phase.

In fact, said gel, once it is passed through the layer 14 which willpermanently be buried in the gel, is able to adhere to the structurewhich has to be made impervious to water, thereby realizing astructure/leakproofing matrix which prevents the migration of water tothe concrete/leakproofing interface.

Moreover, the layer 14 gives a mechanical protection to the underlyingwater-soluble film 12, notably during the panel transport and/orassemblying operations.

According to the present invention the layers constituting the panel 10are joined together either by using a water-soluble glue havingphysical-chemical characteristics equivalent to those of films 12 and13, or by using a non water-soluble glue, provided that in this casesaid non water-soluble glue is not uniformly distributed on the panelsurface.

Once said layers 11, 12, 13, 14 and 15 are superimposed to each otheraccording to the above mentioned sequence, the panel 10 is completed bysealing the longitudinal and cross edges of the films 12 and 13 bythermic welding or by glueing by means of water-soluble and/or nonwater-soluble glues, thereby obtaining a handy panel which can be easilyand efficiently laid.

A further form of embodiment according to the present invention providesfor the production of said leakproofing article in a kerb shape 20 (seeFIG. 2), according to which pure bentonite in powder form 30 is filledinside of an envelope 21 of cylindrical shape having a diameteradvantageously lower than 4 cm.

According to a form of embodiment of the invention, the bentonite isused as powder and it has to be considered pure since it does notcontain any physical binders or mixtures.

Said envelope 21 is produced in water-soluble plastic material,advantageously HYDROLENE® LTF, and has the capability to be leakproofingfor a predetermined period of time in order to suitably delay thebentonite hydration process.

The longitudinal 22 and cross 23 edges of the plastic materialconstituting the envelope 21 are coupled and joined together by thermalwelding, exactly in the same way as it occurs for the panel-shaped formof embodiment 10.

Said kerb 20 can be easily laid in use by means of an assemblying andfastening element 24 (see FIG. 3), said element being advantageouslymade of thermoformable plastic material and having a fastening sidesurface 25.

The fastening operation can be easily realized using nails or rivets 26operating on the fastening surface 25.

Generally, said assemblying element 24 is constituted by an extrudedpolyethylene grate of a prefixed length in order to allow a practicaland safe introduction of the kerb 20 inside of the reinforcement elementof the building structure.

The invention has been previously described with reference to someparticular forms of embodiment.

However, it is apparent that the invention is not limited to these formsof embodiment, but it comprises several variants falling within therange of technical equivalences, as defined by the appended claims.

For instance, it is possible to produce a panel comprising a corrugatedcard-board which is filled with a leakproofing material, advantageouslybentonite, said cardboard being coated on both sides by kraft paper,said kraft paper being then coated by a film made of a syntheticmaterial the water-solubility of which is kept under control, said filmbeing finally coated with a kraft paper layer on both sides or with akraft paper layer on the outer side and with a non-woven fabric and/orfabric layer on the side which directly comes into contact with thestructure to be made impervious to water.

What is claimed is:
 1. A manufactured article impervious to water, to beused in the building industry, comprising a water swellable bentonitelayer, said water swellable bentonite layer is covered with a plasticfilm said plastic film being made of approximately 75% by weightpolyvinylalcohol polymer mixed with approximately 25% by weight polyolicplasticisers and additives, wherein said polyolic plasticisers arealiphatic, high boiling point and are selected among those comprisingnatural or synthetic alcohols and/or alcohol ethers with three carbonatoms in their structures, at least two primary or secondary alcoholicfunctional groups, said plastic film being water soluble and taking apredetermined period of time to dissolve, said period of time being inthe range between 1 and 30 days.
 2. A manufactured article according toclaim 1, wherein said article is shaped as a panel.
 3. A manufacturedarticle according to claim 2, wherein said panel comprises an innerlayer made of biodegradable kraft paper and having a honey-comb shape inorder to contain said bentonite layer, said inner layer being coupled onboth sides to said plastic film.
 4. A manufactured article according toanyone of claims 1, 2 or 3, wherein said film is further coupled to aprotecting layer made of fabric and/or biodegradable kraft paper.
 5. Amanufactured article according to anyone of claims 1, 2 or 3, whereinthe solubility of the polyvinylalcohol decreases when the ratio betweenthe alcohol groups and the acetyl groups in the polymer is higher than95/5.
 6. A manufactured article according to anyone of claims 1, 2 or 3,wherein the layers constituting said article are joined together byglueing.
 7. A manufactured article according to claim 6, wherein theused glue is water-soluble.
 8. A manufactured article according toanyone of 1, 2 or 3, wherein longitudinal and cross edges of said filmare sealed by thermal welding and/or glueing.
 9. A manufactured articleaccording to claim 1, wherein said article is kerb shaped.
 10. Amanufactured article according to claim 9, wherein said kerb comprises acylindrical envelope made of said plastic film, wherein said envelopecontains said bentonite layer.
 11. A manufactured article according toanyone of claims 9 or 10, wherein said kerb is laid by means offastening elements made of thermoformable plastic material.
 12. Amanufactured article according to anyone of claims 9 or 10, wherein saidkerb has a diameter equal or lower than 4 cm.
 13. A manufactured articleimpervious to water, to be used in the building industry, comprising awater swellable bentonite layer, said water swellable bentonite layer iscovered with a plastic film, said plastic film being made ofapproximately 75% by weight polyvinylalcohol polymer mixed withapproximately 25% by weight polyolic plasticisers and additives, thepolyvinyl alcohol having a high molecular weight and an averagehydrolysis degree greater than about 80%, said film being water solubleand taking a predetermined period of time to dissolve, said period oftime being in the range between 1 and 30 days.
 14. A manufacturedarticle as claimed in claim 13 wherein at least one of the polyvinylalcoholic polymers or polyolic platicisers and additives comprisesnatural or synthetic alcohols and/or alcohol ethers with at least threecarbon atoms and two primary or secondary alcoholic functional groups,and wherein the polyolic plasticiser has a aliphatic high boiling point.15. A manufactured article impervious to water, to be used in thebuilding industry, comprising a water swellable bentonite layer, saidwater swellable bentonite layer is covered with a plastic film, saidplastic film being made of approximately 75% by weight polyvinylalcoholpolymer mixed with approximately 25% by weight polyolic plasticisers andadditives, wherein the ethynol/acetile monomer ratio in the polyvinylalcohol is selected such that said film is water soluble and takes apredetermined period of time to dissolve, said period of time being inthe range between 1 and 30 days.
 16. A manufactured article imperviousto water, to be used in the building industry, comprising a waterswellable bentonite layer, said water swellable bentonite layer iscovered with a water soluble plastic film, said plastic film being madeof approximately 75% by weight polyvinylalcohol polymer mixed withapproximately 25% by weight polyolic plasticisers and additives, whereinthe solubility rate is determined on the ratio of the amount of thealcohol group to the amount of acetile group in the polymer such thatsaid film takes a predetermined period of time to dissolve, said periodof time being in the range between 1 and 30 days.
 17. A manufacturedarticle according to claims 13, 14, 15 or 16, wherein said articlefurther comprises an inner layer made of biodegradable kraft paper andhaving a honey-comb shape in order to contain said bentonite layer, saidinner layer being coupled on both sides to said plastic film.